A known radar system has a wide beam transmit antenna to illuminate a wide area with a radar pulse. The signal is scattered by the ground, resulting in an echo signal being received by the radar system. The echoes from ground points closer to the radar system are received before those from ground points further from the radar system. Typically, the time delay between echoes from a near ground point and a far ground point will exceed the duration of the transmitted pulse. Thus, at any one point in time echoes are received from only part of the illuminated ground area. The radar echo can be received using a narrower antenna beam than required for transmission. The narrower beam is required to scan across the ground area with time, to track the motion of the source of the echo signal.
It is known to scan the receive beam using switched delay lines. A phased array antenna comprises a plurality of antenna sub arrays. Each of the antenna sub arrays forming the array has switched delay lines through which the radar signal is received. The switched delay lines introduce a delay, which can be selected between discrete values to scan the receive beam. However, switching the delay requires a finite amount of time, typically tens of nanoseconds. During switching, the received beam characteristics are indeterminate. In addition, the receive beam must be wide enough to ensure it always covers the instantaneous echo extent. A point on the ground will see a different point in the received beam gain pattern each time the beam scan switches. The switching point in the echo will be different for different ground locations. Thus, gain compensation over the pulse duration is difficult, degrading the impulse response function in a direction away from the radar system. It is also known to use a digital beam former which can separately receive and digitise the signals from each sub array of the array, for example from FRONT END TECHNOLOGY FOR DIGITAL BEAMFORMING SAR Christoph Heer, Christian Fischer, Christoph Schaefer IGARSS 2008. The digital beam former then digitally applies the required delay/phase shifts, and digitally combines the signals. Each sub array (or phase centre) requires its own anti-alias filter and digitiser, which must be matched and sample timing synchronised over the signal chains for each phase centre. This results in a system of considerable complexity.